Cerebral vascular contraction, a symptom that a blood vessel of a brain narrows, appears four to 14 days after the onset of subarachnoid hemorrhage. It is believed that almost of all the cases of subarachnoid hemorrhage are accompanied by the symptom. When cerebral vascular contraction develops and a blood vessel of a brain narrows, often a cerebral infraction is caused. Thus, it is required to early and reliably identify a contraction portion of a cerebral blood vessel.
As a method for determining whether cerebral vascular contraction is present, a transcranial Doppler examination is conducted. In this examination, if an average blood flow rate of a horizontal segment of a middle cerebral artery is higher than usual, development of vascular contraction is suspected. Then, an examination that uses a constant medium or MR (magnetic resonance) is carried out to determine whether the vascular contraction has developed. Examples of determining examinations include three-dimensional CT angiography (CTA) and CT perfusion (CTP) that use constant media, MRA (Magnetic Resonance Angiography) and MRI (Magnetic Resonance Imaging) that use magnetic resonance, and SPECT (Single Photon Emission Computed Tomography). One or more images photographed in these examinations are used for determination. In particular, since combined use of CTA and CTP is low-invasive to patients as well as this examination is performed quickly and easily, attention is being paid to the usefulness.
CTP can determine a perfusion value of blood flowing from a blood vessel into capillary vessels in tissue. As perfusion values, values indicating blood circulation of capillary vessels in tissue can be used. Such values include a cerebral blood flow (CBF) per unit volume in cerebral tissue and per unit time, a cerebral blood volume (CBV) being a blood amount per unit volume in cerebral tissue, a mean transit time (MTT) of a blood flow in cerebral tissue, and the like. Each of CBF, CBV and MTT can be calculated based on variations over time in concentration of an injected constant medium in cerebral artery and cerebral tissue parts.
In CTA, an observer moves an image to search for a contraction portion without information of a position of a contraction in cerebral blood vessels. Therefore, the observer may overlook a contraction portion in dense cerebral blood vessels. Also, depending on a display condition of CTA, a contraction portion of a cerebral blood vessel may not be seen on a CTA image. Also, conventionally, even if CTA and CTP are combined with each other, a vascular shape and a blood circulation condition are not displayed on a screen at the same time, so that it is challenging to determine a position of a vascular contraction that causes a deteriorated condition of blood circulation. Also, if both examination images of CTA and CTP are simply arranged and displayed, when a risk of the onset of cerebral infraction at the vascular contraction portion is evaluated, the evaluation may depend on the observer's subjective, and thereby quantitative evaluation cannot be done.